Compounds having two or more thiol groups in one molecule readily react with epoxy resins, urethane resins or the like by mixing them with such resins and become cured products, and therefore, they have been widely used for sealing materials, coating materials, adhesives, etc. For example, various polythiol-based curing agents are described as low-temperature curing agents on page 204 of “Sosetsu Epokishi Jushi (General Reviews Epoxy Resins)” (Vol. 1, Basic Edition, published on Nov. 19, 2003). The conventional polythiol-based epoxy curing agents, however, have a defect that when such a curing agent is mixed with an epoxy compound and a curing assistant at ordinary temperature to form an epoxy resin composition, the pot life of the composition is as short as 3 minutes to 5 minutes and curing is initiated in the course of preparation of the composition, though the curing agent shows excellent curability at low temperatures.
Moreover, there is a problem that storage stability cannot be obtained because the thiol group has high reactivity to various functional groups.
As the compound having a thiol group, which is used for a curing agent for epoxy resins, a compound having a primary thiol group has been heretofore used. The compound having a primary thiol group, however, has a problem that its pot life is so short that the working conditions are restricted, though curing of the compound is accelerated by an amine catalyst.
(1) In Japanese Patent Laid-Open Publication No. 269203/1996, there is disclosed a thiol group-containing polyether polymer obtained by allowing a halogen-terminated polyether polymer which is obtained by addition of epihalohydrin to a polyol having a polyether moiety in the main chain and having 3 or more hydroxyl groups at the ends to react with an alkali hydrosulfide and/or an alkali polysulfide in amides.
It is indicated that an epoxy resin composition containing this polyether polymer has favorable curability, but when this epoxy resin composition is used as a curing agent for epoxy resins, the pot life is so short that curing is initiated while the epoxy resin and a thiol compound that is a curing agent are being mixed with each other, and hence, the working conditions are restricted. Moreover, there is no description of storage stability of the epoxy resin composition, and in the case of long term storage, there is a fear that curing is initiated during storage.
(2) An epoxy resin curing composition using a hetero ring-containing compound, which is described in International Publication No. WO99/54373, has low-temperature curability and rapid curability at ordinary temperature, but the workability is poor because its pot life is short.
(3) In Japanese Patent Publication No. 21693/1992, there is disclosed a liquid polysulfide polymer for epoxy resin curing, which is a copolymer obtained by allowing a mixture of a tri- or tetrafunctional alkyl halide and a bifunctional alkyl halide to react with an alkali polysulfide, the amount of said polyfunctional monomers in the polymerization being 20 to 60% by mol based on all the monomers, and which contains 2 to 30% by mass of end thiol groups. It is indicated that by using this liquid sulfide polymer in combination with an amine, the curing rate becomes higher as compared with that of conventional polysulfide polymers. However, the pot life is short, and the resulting cured product has a problem of workability because it develops odor, though it is excellent in impact resistance, chemical resistance, etc.    Patent document 1: Japanese Patent Laid-Open Publication No. 269203/1996    Patent document 2: International Publication No. WO99/54373    Patent document 3: Japanese Patent Publication No. 21693/1992